RM core inductor calculations

Ferrite cores for inductors have a wide variety of species. E-shaped, U-shaped, pot cores of various modifications.  Such cores are also used with the powder iron materials. Calculation of the coils with any ferromagnetic core uses method calculation with a special parameter AL—inductance factor of the core.

The basic calculation formula:

• L -  inductance (nH)
• A_L - inductance factor of the core (nH/turns square)
• N - number of turns of the coil

The parameter A_L can be found in the manufacturer specifications for each species of core.

However, not all manufacturers publish A_L values ​​for their cores. In this case, you can calculate such a coil if you know the physical dimensions of the core and its magnetic permeability.
The method used Coil64 is recommended for the calculation of the dimensional parameters of RM cores and is in accordance with IEC Publication 205, “Calculation of Effective Parameters of Magnetic Piece Parts.” For this method of calculating the dimensional parameters of RM cores, the RM core set is substituted by an ideal toroidal core such that a coil wound on that toroid would give exactly the same electrical performance as a coil with same number of turns placed on the RM core set. The dimensional parameters of that substitute toroid are called effective parameters. These are indicated by the suffix “e” added to the symbol.

1. Effective magnetic path length l_e mm
2. Effective cross-sectional area A_e mm²
3. Effective core volume V_e mm³

For the purpose of the calculation of the dimensional parameters, the closed magnetic circuit of the RM core is divided into five sections. For each section, the area, and magnetic flux path length are determined.

1. Area of the legs A₁ is:
2. For two plates with the flux length l₂ = l'₂ + l"₂:where:Areas A₇, A₈, paths L_min and L_max can be calculated according to the IEC 60205 standard.
3. Area of the center limb A₃:
4. Mean flux path length at corners:
5. Cross-sectional areas associated with l₄ and l₅:
6. The core constants for the total magnetic circuit of the RM core are:
7. Now we can find the effective parameters l_e [mm], A_e [mm²], V_e [mm³] of the RM core by the following formulas:
8. To take into account the effect of the gap in the magnetic circuit, that has a high reluctance, the gapped core is substituted by a non-gapped one, with the same electrical performance and with the same number of turns. The magnetic permeability of such a core is called the effective magnetic permeability:where µ_r is the actual magnetic permeability of the core. When the coil is operating in the small signal mode, this value can be taken equal to the value of the initial magnetic permeability µ_i.
9. At the finish, we find the inductance of the RM core coil set:where:

• L - inductance [mH];
• µ₀ = 4 π · 10^-7 - the magnetic constant [H/m];
• µ_e - effective magnetic permeability of the core;
• N - the numbers of turns;
• A_e - effective cross-sectional area of the core [mm²];
• l_e - effective magnetic flux path length of the core [mm];

This calculation ignore the effect of spring recesses and stud recesses. They have a considerable influence, especially for smaller cores. Moreover, the µ_i spread of the core is quite high and depends on the temperature. The core dimensions are subject to manufacturing tolerances. The leakage of joint of the core halves is of great importance. Even with a very small gap, of the order of 0.01 mm, the effective magnetic permeability is much less than the initial one. Therefore, due to these factors,, the calculation of the coils in RM core by this method has a considerable error of up to ±30% and above and can be useful only for cores with unknown A_L. For more accurate calculations, it is necessary to use the A_L factor, if you found it in the pot core Datasheet.

Related links:

1. Calculation of coils with ferromagnetic cores (general inquiries);
2. Guide for calculation of the effective parameters of magnetic piece parts - Bureau of Indian Standards (BIS) p.16…17;